processes - swarthmore collegenewhall/cs31/resources/week10-pr… · how are processes created?...

Processes

CS31, Week 10Prof. Jason WatermanNovember 15, 2012

Jason Waterman, Swarthmore College 2

Topics for today

•The UNIX process abstraction•Process lifecycle

•Creating processes: forking•Running new programs within a process•Terminating and reaping processes

•Signaling processes

What is a process?

•A process is the OS's abstraction for execution•A process is an instance of a program in execution.•i.e., each process is running a program; there may be

many processes running the same program

What is a process?

•A process is the OS's abstraction for execution•A process is an instance of a program in execution.•i.e., each process is running a program; there may be

many processes running the same program

•A process provides•Private address space• Through the mechanism of virtual memory!

•Illusion of exclusive use of processor

Jason Waterman, Swarthmore College

Process context

•Process context is the state that the operating system needs to run a process

Process context

•Process context is the state that the operating system needs to run a process•1) Address space• The memory that the process can access

• Consists of various pieces: the program code, global/static variables, heap, stack, etc.

Process context

•2) Processor state• The CPU registers associated with the running process

• Includes general purpose registers, program counter, stack pointer, etc.

Process context

•2) Processor state• The CPU registers associated with the running process

• Includes general purpose registers, program counter, stack pointer, etc.

•3) OS resources• Various OS state associated with the process

• Examples: page table, file table, network sockets, etc.

Context switches

• Multiple processes can run simultaneously.• On a single CPU system, only one process is running on the CPU at a time.

• But can have concurrent execution of processes

• On a multi-CPU (or multi-core) system, multiple processes can run in parallel.

• The OS will timeshare each CPU/core, rapidly switching processes across them all.

Context switches

• Switching a CPU from running one process to another is called a context switch.• (1) Save the context of the currently running process,

• (2) Restore the context of some previously preempted process

• (3) Resume execution of the newly restored process

Context switches

• Switching a CPU from running one process to another is called a context switch.• (1) Save the context of the currently running process,

• (2) Restore the context of some previously preempted process

• (3) Resume execution of the newly restored process

• Deciding when to preempt current process and restart previously preempted process is known as scheduling• Performed by part of the OS called a scheduler

Process IDs

•Each process has a unique positive process ID (PID)

•getpid returns current process’s PID

•getppid returns PID of parent of current process

pid_t getpid(void);pid_t getppid(void);

Process states

• At any moment, a process is in one of several states:• Ready:

• Process is waiting to be executed

• Running:

• Process is executing on a CPU

• Stopped:

• Process is suspended (due to receiving a certain signal) and will not be scheduled

• More on signals soon...

• Waiting (or sleeping or blocked):

• Process is waiting for an event to occur, such as completion of I/O, timer, etc.

• Why is this different than “ready” ?

• Terminated:

• Process is stopped permanently, e.g., by returning from main, or by calling exit

• As the process executes, it moves between these states• What state is the process in most of the time?

Process lifecycle

Stopped

Blocked

Terminated

Running

Process lifecycle

new process

Stopped

Blocked

Terminated

Running

Process lifecycle

new process

Stopped

Blocked

Terminated

Running

Process lifecycle

new process

Waiting for event

e.g., I/O, tim

er, ...Ready

Stopped

Blocked

Terminated

Running

Process lifecycle

new process Event occurs

Waiting for event

e.g., I/O, tim

er, ...Ready

Stopped

Blocked

Terminated

Running

Process lifecycle

SIGSTOP signal

Waiting for event

e.g., I/O, tim

er, ...Ready

Stopped

Blocked

Terminated

Running

Process lifecycle

SIGSTOP signal

Waiting for event

e.g., I/O, tim

er, ...

SIGCONT signal

Stopped

Blocked

Terminated

Running

Process lifecycle

new process

Terminate

Event occurs

SIGSTOP signal

Waiting for event

e.g., I/O, tim

er, ...

SIGCONT signal

Terminate

Stopped

Blocked

Terminated

Running

Topics for today

How are processes created?

• Typically, new process is created when user runs a program• E.g., Double-click an application, or type a command at the shell

• In UNIX, starting a new program is done by some other process• The shell is a process itself!

• So are the Dock and Finder in MacOS (a variant of UNIX)

• One process (e.g., the shell) is creating another process (the command you want to run)• This is called forking

• Every process has a parent process

• Chicken-and-egg problem: How does first process get created?

• Chicken-and-egg problem: How does first process get created?• At boot time, the OS creates the first process, called init, which is

responsible for starting up many other processes

fork: Creating New Processes

•int fork(void)•creates a new process (child process) that is identical to the

calling process (parent process)

•returns 0 to the child process

•returns child’s process ID (pid) to the parent process

if (fork() == 0) { printf("hello from child\n");} else { printf("hello from parent\n");}

•returns 0 to the child process

•returns child’s process ID (pid) to the parent process

•Fork is interesting (and often confusing) because it is called once but returns twice

if (fork() == 0) { printf("hello from child\n");} else { printf("hello from parent\n");}

• Parent and child process both run the same program.• Only difference is the return value from fork()

• Child’s address space starts as an exact copy of parent’s• They do not share the memory – instead they each have a private copy.

• Also have the same open files with the same offsets into the files.

• Includes stdin, stdout, and stderr

Fork Example #1

void fork1(){ int x = 1; pid_t pid = fork(); if (pid == 0) { printf("Child has x = %d\n", ++x); } else { printf("Parent has x = %d\n", --x); } printf("Bye from process %d with x = %d\n", getpid(), x);}

Fork Example #1

void fork1(){ int x = 1; pid_t pid = fork(); if (pid == 0) { printf("Child has x = %d\n", ++x); } else { printf("Parent has x = %d\n", --x); } printf("Bye from process %d with x = %d\n", getpid(), x);}

Parent has x = 0Bye from process 9991 with x = 0Child has x = 2Bye from process 9992 with x = 2

•Key Points•Both parent and child can continue forking

Fork Example #2

void fork2(){ printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n");}

Fork Example #2

void fork2(){ printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n");} L0

Fork Example #2

void fork2(){ printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n");} L0 L1

Fork Example #2

void fork2(){ printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n");} L0 L1

ByeBye

Fork Example #3

void fork3(){ printf("L0\n"); fork(); printf("L1\n"); fork(); printf("L2\n"); fork(); printf("Bye\n");}

Fork Example #3

ByeBye

ByeByeByeBye

ByeBye

Fork Example #4

void fork4(){ printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } } printf("Bye\n");}

Fork Example #4

ByeBye

Fork Example #5

void fork5(){ printf("L0\n"); if (fork() == 0) { printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork(); } } printf("Bye\n");}

Fork Example #5

L0 Bye

Fork Example #5

L0 Bye

L1 Bye

Fork Example #5

L0 Bye

Starting new programs

•How do we start a new program, instead of copying the parent?

•Use the UNIX execve() system call

•int execve(const char *filename, char *const argv [], char *const envp[]);•filename: name of executable file to run

•argv: Command line arguments

•envp: environment variable settings (e.g., $PATH, $HOME, etc.)

•execve() does not fork a new process!•Rather, it replaces the address space and CPU state of

the current process•Loads the new address space from the executable file

and starts it from main()•So, to start a new program, use fork() followed by execve()

Using fork and exec

int main(int argc, char **argv) { if (fork() == 0) { /* Child process */ char *newargs[3]; printf(“Hello, I am the child process.\n”); newargs[0] = “/bin/echo”; /* Convention! Not required!! */ newargs[1] = “some random string”; newargs[2] = NULL; /* Indicate end of args array */ if (execv(“/bin/echo”, newargs)) { printf(“warning: execv returned an error.\n”); exit(-1); } printf(“Child process should never get here\n”); exit(42); }}

Stephen Chong, Harvard University

Intermission

Stephen Chong, Harvard University

Intermission

You are a prisoner sentenced to death. The Emperor offers you a chance to live by playing a simple

game. He gives you 50 black marbles, 50 white marbles and 2 empty bowls. He says, "Divide these 100 marbles into these 2 bowls,

any way you like so long as you use all the marbles. Then I will blindfold you and mix the bowls around. You then can choose one bowl and remove

ONE marble. If the marble is WHITE you will live, but if the marble is BLACK... you will die."

How do you divide the marbles up so that you have the greatest probability of choosing a WHITE

marble?

http://www.braingle.com/

Topics for today

Terminating a process

•A process terminates for one of 3 reasons:•(1) return from the main() procedure

•(2) call to the exit() function

•(3) receive a signal whose default action is to terminate

exit: Destroying Process

• void exit(int exit_status)

• void exit(int exit_status)•Exits a process with specified exit status.

•By convention, status of 0 is a “normal” exit, non-zero indicates an error of some kind.

•atexit() registers functions to be executed upon exit.void cleanup(void) { printf("cleaning up\n");}

void fork6() { atexit(cleanup); fork(); exit(0);}

Zombies

•When a process terminates (for whatever reason) OS does not remove it from system immediately

Zombies

•Process stays until it is reaped by parent

Zombies

•Process stays until it is reaped by parent•When parent reaps a child process, OS gives the parent the exit

status of child, and cleans up child

Zombies

•Process stays until it is reaped by parent•When parent reaps a child process, OS gives the parent the exit

status of child, and cleans up child

•A terminated process that has not been reaped is called a zombie process

•How do you reap a child process?

Zombies

wait: Synchronizing with Children

• int wait(int *child_status)• Suspends parent process until one of its children terminates

• Return value is the pid of the child process that terminated

• if child_status != NULL, it will point to the child's return status

• int wait(int *child_status)• Suspends parent process until one of its children terminates

• Return value is the pid of the child process that terminated

• if child_status != NULL, it will point to the child's return status

• child_status can be accessed using several macros:• WIFEXITED(child_status) == 1 if child exited due to call to exit()• WEXITSTATUS(child_status) gives the return code passed to exit()• WCOREDUMP(child_status) == 1 if child dumped core.

• And others (see “man 2 wait”)

void fork9() { int child_status;

if (fork() == 0) { printf("HC: hello from child\n"); } else { printf("HP: hello from parent\n"); wait(&child_status); printf("CT: child has terminated\n"); } printf("Bye\n"); exit();}

HC Bye

CT Bye

• wait() returns status of exited children in arbitrary order.

What if multiple child processes exit?

• wait() returns status of exited children in arbitrary order.#define N 10

void fork10(){ pid_t pid[N]; int i, child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminate abnormally\n", wpid); }}

• wait() returns status of exited children in arbitrary order.#define N 10

void fork10(){ pid_t pid[N]; int i, child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminate abnormally\n", wpid); }}

linux> ./fork10Child 2625 terminated with exit status 195Child 2627 terminated with exit status 197Child 2626 terminated with exit status 196Child 2624 terminated with exit status 194Child 2623 terminated with exit status 193Child 2622 terminated with exit status 192Child 2621 terminated with exit status 191Child 2620 terminated with exit status 190...

waitpid(): Waiting for a specific process

•pid_t waitpid(pid_t child_pid, int *status, int options)• Causes parent to wait for a specific child process to exit.

• Most general form of wait•child_pid > 0: wait for specific child to exit

•child_pid = -1: wait for any child to exit

• return value is PID of child process

•options can be used to specify if call should return immediately (with return value of 0) if no terminated children, and also whether we are interested in stopped processes

•status encodes information about how child exited (or was stopped)

•pid_t waitpid(pid_t child_pid, int *status, int options)

void fork11(){ pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = waitpid(pid[i], &child_status, 0); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminated abnormally\n", wpid); }

linux> ./fork11Child 3064 terminated with exit status 100Child 3065 terminated with exit status 101Child 3066 terminated with exit status 102Child 3067 terminated with exit status 103Child 3068 terminated with exit status 104Child 3069 terminated with exit status 105Child 3070 terminated with exit status 106...

Back to the zombies...

void zombie(){ if (fork() == 0) { /* Child */ printf("Terminating Child, PID = %d\n", getpid()); exit(0); } else { printf("Running Parent, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ }}

•Zombie example

linux> ./zombie &

•Zombie example

linux> ./zombie &[1] 6639Running Parent, PID = 6639Terminating Child, PID = 6640

•Zombie example

linux> ./zombie &[1] 6639Running Parent, PID = 6639Terminating Child, PID = 6640linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 zombie 6640 ttyp9 00:00:00 zombie <defunct> 6641 ttyp9 00:00:00 pslinux>

•Zombie example

•ps shows child process as “defunct”

linux> ./zombie &[1] 6639Running Parent, PID = 6639Terminating Child, PID = 6640linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 zombie 6640 ttyp9 00:00:00 zombie <defunct> 6641 ttyp9 00:00:00 pslinux>

Orphans

•So bad things happen if the parent does not wait for the child...

Orphans

•If the child exits first, child becomes a zombie

Orphans

•If the parent exits first, the child becomes an orphan.

Orphans

•If the parent exits first, the child becomes an orphan.•Problem: All processes (except for init) need a parent process.

Orphans

•Orphan processes “adopted” by init (PID 1 on most UNIX systems)

Orphans

•Orphan processes “adopted” by init (PID 1 on most UNIX systems)

•If child subsequently terminates, it will be reaped by init• init reaps zombie orphans...

Nonterminating Child Example

•Child process still active even though parent has terminated

•Must kill explicitly, or else will keep running indefinitely

void fork8(){ if (fork() == 0) { /* Child */ printf("Running Child, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ } else { printf("Terminating Parent, PID = %d\n", getpid()); exit(0); }}

linux> ./fork8Terminating Parent, PID = 6675Running Child, PID = 6676

linux> ./fork8Terminating Parent, PID = 6675Running Child, PID = 6676linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 fork8 6677 ttyp9 00:00:00 ps

linux> ./fork8Terminating Parent, PID = 6675Running Child, PID = 6676linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 fork8 6677 ttyp9 00:00:00 pslinux> kill 6676

linux> ./fork8Terminating Parent, PID = 6675Running Child, PID = 6676linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 fork8 6677 ttyp9 00:00:00 pslinux> kill 6676linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6678 ttyp9 00:00:00 ps

Zombie orphan

•Zombie example

Zombie orphan

•Zombie example

linux> ./zombie &[1] 6639Running Parent, PID = 6639Terminating Child, PID = 6640

Zombie orphan

•Zombie example

linux> ./zombie &[1] 6639Running Parent, PID = 6639Terminating Child, PID = 6640linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 zombie 6640 ttyp9 00:00:00 zombie <defunct> 6641 ttyp9 00:00:00 ps

Zombie orphan

•Zombie example

•ps shows child process as “defunct”

Zombie orphan

•Zombie example

•ps shows child process as “defunct”•Killing parent allows child to be reaped

Zombie orphan

•Zombie example

linux> ./zombie &[1] 6639Running Parent, PID = 6639Terminating Child, PID = 6640linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 zombie 6640 ttyp9 00:00:00 zombie <defunct> 6641 ttyp9 00:00:00 pslinux> kill 6639[1] Terminated

Zombie orphan

•Zombie example

linux> ./zombie &[1] 6639Running Parent, PID = 6639Terminating Child, PID = 6640linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 zombie 6640 ttyp9 00:00:00 zombie <defunct> 6641 ttyp9 00:00:00 pslinux> kill 6639[1] Terminatedlinux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6642 ttyp9 00:00:00 ps

Zombie orphan

Topics for today

Signals

• Unix provides a mechanism to allow processes and OS to interrupt other processes

• A signal is small message to notify a process of some system event• These messages not normally visible to the program

• e.g.,

Signals

• e.g.,

Signals

• e.g.,

ID Name Default Action Corresponding Event

2 SIGINT Terminate Interrupt (e.g., ctl-c from keyboard)

9 SIGKILL Terminate Kill program (cannot override or ignore)

11 SIGSEGV Terminate & Dump Segmentation violation

14 SIGALRM Terminate Timer signal

17 SIGCHLD Ignore Child stopped or terminated

19 SIGSTOP Stops process Process asked to stop

18 SIGCONT Continue process Process asked to continue

Signals

• e.g.,

Signals

• e.g.,

Signals

• e.g.,

Signals

• e.g.,

Signals

• e.g.,

Signals

• e.g.,

Signals

• e.g.,

Signals

• e.g.,

Constant values may vary between platforms!

Signal concepts

•Two distinct steps to transfer a signal:

Signal concepts

•Two distinct steps to transfer a signal:•(1) OS sends (delivers) signal to destination process

Signal concepts

•Two distinct steps to transfer a signal:•(1) OS sends (delivers) signal to destination process• either because of some system event, or because explicitly requested via kill function

Signal concepts

•(2) Process receives signal (i.e., forced by OS to react to signal in some way)

Signal concepts

•(2) Process receives signal (i.e., forced by OS to react to signal in some way)• Process can react in one of three ways:

Signal concepts

•(2) Process receives signal (i.e., forced by OS to react to signal in some way)• Process can react in one of three ways:‣ignore signal (i.e., do nothing)

Signal concepts

•(2) Process receives signal (i.e., forced by OS to react to signal in some way)• Process can react in one of three ways:‣ignore signal (i.e., do nothing)‣terminate (maybe dumping core)

Signal concepts

•(2) Process receives signal (i.e., forced by OS to react to signal in some way)• Process can react in one of three ways:‣ignore signal (i.e., do nothing)‣terminate (maybe dumping core)‣catch a signal with a signal handler function

Signal concepts

•Signal sent but not yet received is pending•At most one signal of each type is pending•Signals are not queued! • If process has pending signal of type k, then subsequent signals of type k are discarded

Signal concepts

•Process can block receipt of certain signals. •Blocked signals will be pending until process unblocks

Signal concepts

•Process can block receipt of certain signals. •Blocked signals will be pending until process unblocks

•Any signal received at most once

Pending and blocking signals

• OS maintains pending and blocked bit vectors for each process

• OS maintains pending and blocked bit vectors for each process• pending represents set of pending signals

• OS sets bit k of pending when signal of type k is delivered

• OS clears bit k of pending when signal of type k is received

0 0 0 0 0

...pending:

0 0 0 0 0

...pending: 1

0 0 0 0 0

...pending: 1 1

• blocked represents set of signals process has blocked

• Can be set and cleared using sigprocmask function

0 0 0 0 0

...pending: 1 1

0 0 0 0 0

...pending: 1

0 0 0 0 0 ...blocked:

0 0 0 0 0

...pending: 1

0 0 0 0 0 ...blocked: 0 0 10 1

• For a process, OS computes pnb = pending & ~blocked

0 0 0 0 0

...pending: 1

0 0 0 0 0 ...blocked: 0 0 10 1

0 0 0 0 0

...pending: 1

0 0 0 0 0 ...blocked: 0 0 10 1

0 1 0 0 0

...pnb:

• If pnb == 0 then no signals to be received

0 0 0 0 0

...pending: 1

0 0 0 0 0 ...blocked: 0 0 10 1

0 1 0 0 0

...pnb:

• If pnb == 0 then no signals to be received

• If pnb != 0 then OS chooses a signal to be received, and triggers some action by process

0 0 0 0 0

...pending: 1

0 0 0 0 0 ...blocked: 0 0 10 1

0 1 0 0 0

...pnb:

Sending signals with kill

•kill programs sends an arbitrary signal to a process

•E.g., kill -9 24818 sends SIGKILL to process 24818

•Also a function: kill(pid_t p, int signal)

•Can send a signal to a specific process, or all processes in a process group•Every process belongs to a process group

•Read textbook for more info

Default actions

•Each signal type has a predefined default action•One of

•The process terminates•The process terminates and dumps core•The process stops (until restarted by a SIGCONT

signal)•The process ignores the action

Signal handlers

•signal(int signum, handler_t *handler)•Overrides default action for signals of kind signum

•Different values for handler•SIG_IGN: ignore signals of type signum•SIG_DFL: revert to the default action for signals of type signum•Otherwise, it is a function pointer for a signal handler• Function will be called on receipt of signal of type signum• Referred to as installing handler

• Handler execution is called handling or catching signal

• When handler returns, control flow of interrupted process continues

Signal handler example

void int_handler(int sig) { printf("Process %d received signal %d\n", getpid(), sig);}

int main() { signal(SIGINT, int_handler); while (1) ;}

$ ./signaleg

$ ./signaleg^C

$ ./signaleg^CProcess 319 received signal 2

$ ./signaleg^CProcess 319 received signal 2^C

$ ./signaleg^CProcess 319 received signal 2^C Process 319 received signal 2

$ ./signaleg^CProcess 319 received signal 2^C Process 319 received signal 2^C

$ ./signaleg^CProcess 319 received signal 2^C Process 319 received signal 2^C Process 319 received signal 2

$ kill -9 319

$ ./signaleg^CProcess 319 received signal 2^C Process 319 received signal 2^C Process 319 received signal 2Killed

$ kill -9 319

Signal handlers as concurrent flows

•Signal handlers run concurrently with main program•Signal handler is not a separate process

•Concurrent here means “non-sequential”, as opposed to “parallel”

(2) Control passes !to signal handler !

(3) Signal handler runs!

(4) Signal handler!returns to !

next instruction!

Icurr!Inext!

(1) Signal received by process !

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